T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic cancer that occurs in children as well as adults. Despite intensive therapies, 25% of children and adolescents and 50% of adults with T-ALL will ultimately succumb to the disease. Moreover, the late effects of cancer treatment, including permanent organ damage, hormonal and reproductive dysfunction, and second cancers are a special concern in children as current therapies indiscriminately target all dividing cells. The goal of this proposal is to identify and therapeutically target a pathway that is essential for the growth and survival of T-ALL, the inhibition of which will be considerably less toxic than standard chemotherapy. Class I phosphoinositide 3-kinase (PI3K)/Akt signaling pathway has been reported to be activated in over 40% of cases of T-ALL. Although four distinct class I PI3K isoforms could participate in T-ALL pathogenesis, none have been implicated in this process. Using genetically altered animals and novel small molecule inhibitors, we have identified specific isoforms that appear to be essential for the development and survival of T- ALL. Based on these observations, we propose that it is possible to exploit the addiction of this hematological malignancy to these particular PI3K isoforms as a new therapeutic avenue for T-ALL. To achieve these objectives, we propose the following specific aims: Aim 1. To determine the efficacy and to define the mechanism(s) of action by which 2 novel isoform specific PI3K inhibitors may impact on T-ALL pathogenesis. This will be accomplished by evaluating the effects of these inhibitors in an animal model of PTEN null T-ALL, on human T-ALL cell lines, and on primary patient samples. Emphasis will be placed on interrogating downstream pathways that regulate cell cycle progression, protein synthesis, and cell survival using a combination of biochemical and genetic approaches. Aim 2. To uncover potential PI3K-dependent and independent mechanisms that may result in secondary resistance. This includes identifying resistance-conferring point mutations and alternative signaling pathways using in silico models of drug binding, chemical-induced mutagenesis, and drug-induced resistant disease in animals. Biochemical and genetic analyses will be performed, the latter including gene expression microarrays and array-based comparative genomic hybridization. Aim 3. To determine the effects of isoform specific PI3K inhibitors on the development, peripheral expansion, survival and function of human T cells. This will be accomplished by evaluating drug effects in a unique human thymus/CD34+ stem cell xenograft model.